1. Technical Field
The present invention relates to a hybrid stepping motor of which rotor has a permanent magnet to achieve a high efficiency and is shaped like a gear to realize a very small angular rotation, and more particularly to a hybrid stepping motor which is easy to drive, has a stability at a high speed and ensures a high productivity.
2. Background Art
Today, stepping motors are used in various fields of industry since its rotational angle and rotational speed can be controlled with high precision. Particularly, a hybrid stepping motor is outstanding since it has many advantages.
A typical hybrid stepping motor includes a rotor made from two vertically extending rod-shaped pieces (N and S polarity pieces) and a stator provided with four or five pairs of electromagnet. If the stator has four pairs of electromagnets (eight electromagnets in total), a rotation angle for one stepping motion is 45 degrees (360/8=45) and if the stator has five pairs of electromagnet (ten in total), the rotational angle for one stepping motion is 36 degrees (360/10=36). These eight or ten electromagnets are arranged at constant intervals on an inner wall of the stator in a circumferential direction of the stator.
Generally, it is possible to independently excite the respective pairs of electromagnet. The electromagnets are excited in the order of their positions in the circumferential direction of the stator. The excitation is performed to attract the N and S pole pieces. The stepping motor which requires four stepping motions to rotate the rotor 180 degrees (i.e., the stepping motor having four pairs of electromagnets) is called "four-phase" stepping motor and one which requires five stepping motions (i.e., the stepping motor having five pairs of electromagnets) is called "five-phase" stepping motor. As compared with the four-phase type one, the five-phase stepping motor has a smaller rotation angle for one stepping motion (36 degrees), which results in a higher resolution and stability. However, the five-phase stepping motor does not have electromagnets having a 90-degree relation (ten electromagnets are arranged at 36-degree intervals) so that if one electromagnet attracts the N pole piece, the S pole piece which vertically crosses the N pole piece is attracted by two electromagnets (second and third next electromagnets at 72 and 108 degree positions). Technically, it would be advantageous if the S pole pieces was attracted by a single electromagnet. In the next stepping motion, two electromagnets attract in combination the N pole piece and one electromagnet attracts the S pole piece. Therefore, the five-phase stepping motor requires a complicated combination of excitation. On the other hand, the four-phase stepping motor has eight electromagnets at 45-degree intervals so that when one electromagnets attracts the N pole piece, another electromagnet attracts the S pole piece. However, since the stepping pitch of the four-phase stepping motor is large (45 degrees), the resolution is degraded and the torque stability is also degraded. Accordingly, the four-phase type one and the five-phase type one have complementary features relative to each other.
If a number of electromagnets or phases of the stepping motor increases, the rotation angle for one stepping motion becomes smaller. As a result, the resolution is raised and fluctuation of torque between the steps is reduced. However, there is a limitation on the increment of the number of the electromagnets.
Meanwhile, another type of stepping motor is known in the art: each of the N and S pole pieces of the rotor is shaped like a gear and the electromagnets of the stator have teeth (stator teeth) on their free ends respectively at constant intervals such that the stator teeth face teeth on the N and S pole gears (rotor teeth). Movement corresponding to one tooth pitch of the rotor teeth is caused by four or five stepping motions. As to this type of stepping motor, a number of stepping motions required to rotate the rotor one rotor tooth pitch is called "number of phases". The rotational angle for one stepping motion is obtained by dividing the rotor tooth pitch by the number of phases. The number of phases is the same as a number of excitation patterns.
The hybrid stepping motor having a gear-shaped rotor has a smaller stepping pitch, but the combination or order of excitation to the electromagnets (called "main poles" hereinafter) is the same as the stepping motors having the rod-shaped rotor. In other words, if the rotor is shaped like a gear, the five-phase type has a larger number of phases and its rotation angle for one stepping motion is smaller than the four-phase type, but the five-phase one does not have main poles having a 90-degree relation and accordingly the combination of excitation is complicated as compared with a four-phase type one. Such a complicated combination of excitation is not preferred to the torque efficiency to the electric power and the rotational stability.
There is an arrangement for rationalizing the combination of excitation. The intervals between each adjacent stator teeth are not made equal to each other and the rotational angles for the respective stepping motions are varied to correspond to the unequal intervals. This arrangement also can reduce the stepping pitch. To arrange the stator teeth at unequal intervals, the main pole intervals should be changed or the tooth or teeth on the main pole should be formed with certain discrepancy with respect to the center line of the main pole without changing the intervals of the main poles. However, if the intervals (angle) between the main poles are not uniform, the symmetry around the rotation axis of the stepping motor is deteriorated and the motor surfers from a degraded stability during the high speed rotation, and if the teeth on one main pole are formed with the discrepancy, magnetic fields of uniform intensity cannot be expected even if same power is applied to the main poles since coils are wound around the main poles.
In addition, generally the stepping motors are manufactured in large quantities, but wiring requires human labors. Therefore, it is desired to simplify the manufacturing process. To achieve this, a wiring machine should be used effectively and the stator should have a structure which enables an automatic manufacturing.